A diesel type internal combustion engine involves a problem in exhaust particulates (particulate matters, PM) contained in exhaust gas discharged from an internal combustion engine body. Accordingly, a diesel particulate filter (DPF) is used to trap PM. PM are trapped and accumulated in the DPF, and the PM are periodically burned and removed, so that the DPF is regenerated and the PM trapping capacity of the DPF is restored. Regeneration of the DPF can be made in operation of the internal combustion engine. For instance, the oxidation action of the oxidation catalyst such as platinum is used. The oxidation catalyst is increased in temperature by post injection or retard in ignition timing, so that regeneration of a DPF is started.
Delay in timing of regeneration of the DPF causes fast burn, and advance in the timing of regeneration of the DPF may increase fuel consumption. Accordingly, necessity of regeneration of the DPF is evaluated in accordance with a PM sediment amount (PM amount), which is measured. As the PM amount increases, differential pressure between the front of the DPF and the rear of the DPF increases. Therefore, the PM amount may be calculated in accordance with the differential pressure of the DPF. However, the calculation of the PM amount, which is based on the differential pressure of the DPF, is not necessarily satisfactory in accuracy of measurement in an unsteady operating state. The unsteady operating state includes a state, in which exhaust gas flows through the DPF at a small flow rate, and a transient state. According to JP-A-7-317529 and U.S. Pat. No. 6,758,039 (JP-A-2004-019529), the PM discharge amount is calculated in accordance with an operating state of the internal combustion engine, so that the PM discharge amount is integrated for calculating the PM amount.
Here, when abnormality is caused in flow of exhaust gas due to plugging or breakage in the DPF, the differential pressure of the DPF changes from that in a normal condition, in which exhaust gas normally flows. The abnormality in the DPF can be detected in accordance with the differential pressure. Specifically, when the differential pressure of the DPF is less than a predetermined value, exhaust gas is determined to be leaking due to breakage of the DPF.
When the PM amount is large, abnormality in the DPF can be satisfactory detected, since the differential pressure of the DPF sufficiently changes at the time of breakage. However, when the PM amount is small as immediately after completion of regeneration of the DPF, the differential pressure does not sufficiently change. Accordingly, presence and absence of the abnormality in the DPF may not be correctly detected.